The cell cycle of Tetrahymena i being used as a model system for an inquiry on the effect of replication gene dosage changes on the transcription rate of a specific gene product. The generation time of Tetrahymena is three hours and a synchronous population is obtained by a selection procedure (Wolfe 1973 Exptl. Cell REs. 77, 232). Such a procedure is necessary if disturbance of regulatory mechanisms is to be avoided. Using two different methods, moleclar hybridization and autoradiography, the precise time of replication of the cistrons for ribosomal RNA will be established. The question is, will the rate of transcription of ribosomal RNA be increased in proportion to the increased availability of template, or does a regulatory mechanism exist which controls the rate oftranscription independent of available template. The rate of transcription will be determined by gradient analysis of RNA ssnthesized in a 5 minute period, using H3-uridine as a tracer, and measuring the specific activity of the UTP pool at various stages of the interdivsion period. Synchronous cells will also be used in the induction of the mating reaction to study cellular interactions and gene regulation during the induction of differentiation.